The RORC Rating Office issues two different types of sail numbers:
3 or 4 figure numbers (with an R suffix) eg. GBR123R/GBR1234R: These are issued to the BOAT and by default they stay with the boat when it is sold, unless specific arrangements are made to retain the number.
2 figure "special" numbers (with an R suffix) eg. GBR12R: These are like personal numberplates and are issued to a PERSON rather than the boat. By default the sail number does not transfer with the boat when it is sold, it remains the property of the person who purchased it. However, to avoid confusion we strongly recommend that it is explicitly stated in the sale documents that the sail number is to be retained.
The safest thing to do is always explicitly state in the sale documents if the sail number is to be retained by the previous owner.